NAME¶
CMS_verify, CMS_get0_signers - verify a CMS SignedData structure
SYNOPSIS¶
#include <openssl/cms.h>
int CMS_verify(CMS_ContentInfo *cms, STACK_OF(X509) *certs, X509_STORE *store, BIO *indata, BIO *out, unsigned int flags);
STACK_OF(X509) *CMS_get0_signers(CMS_ContentInfo *cms);
DESCRIPTION¶
CMS_verify() verifies a CMS SignedData structure.
cms is the
CMS_ContentInfo structure to verify.
certs is a set of certificates in
which to search for the signing certificate(s).
store is a trusted
certificate store used for chain verification.
indata is the detached
content if the content is not present in
cms. The content is written to
out if it is not NULL.
flags is an optional set of flags, which can be used to modify the verify
operation.
CMS_get0_signers() retrieves the signing certificate(s) from
cms,
it must be called after a successful
CMS_verify() operation.
VERIFY PROCESS¶
Normally the verify process proceeds as follows.
Initially some sanity checks are performed on
cms. The type of
cms
must be SignedData. There must be at least one signature on the data and if
the content is detached
indata cannot be
NULL.
An attempt is made to locate all the signing certificate(s), first looking in
the
certs parameter (if it is not NULL) and then looking in any
certificates contained in the
cms structure itself. If any signing
certificate cannot be located the operation fails.
Each signing certificate is chain verified using the
smimesign purpose
and the supplied trusted certificate store. Any internal certificates in the
message are used as untrusted CAs. If CRL checking is enabled in
store
any internal CRLs are used in addition to attempting to look them up in
store. If any chain verify fails an error code is returned.
Finally the signed content is read (and written to
out is it is not NULL)
and the signature's checked.
If all signature's verify correctly then the function is successful.
Any of the following flags (ored together) can be passed in the
flags
parameter to change the default verify behaviour.
If
CMS_NOINTERN is set the certificates in the message itself are not
searched when locating the signing certificate(s). This means that all the
signing certificates must be in the
certs parameter.
If
CMS_NOCRL is set and CRL checking is enabled in
store then any
CRLs in the message itself are ignored.
If the
CMS_TEXT flag is set MIME headers for type
text/plain are
deleted from the content. If the content is not of type
text/plain then
an error is returned.
If
CMS_NO_SIGNER_CERT_VERIFY is set the signing certificates are not
verified.
If
CMS_NO_ATTR_VERIFY is set the signed attributes signature is not
verified.
If
CMS_NO_CONTENT_VERIFY is set then the content digest is not checked.
NOTES¶
One application of
CMS_NOINTERN is to only accept messages signed by a
small number of certificates. The acceptable certificates would be passed in
the
certs parameter. In this case if the signer is not one of the
certificates supplied in
certs then the verify will fail because the
signer cannot be found.
In some cases the standard techniques for looking up and validating certificates
are not appropriate: for example an application may wish to lookup
certificates in a database or perform customised verification. This can be
achieved by setting and verifying the signers certificates manually using the
signed data utility functions.
Care should be taken when modifying the default verify behaviour, for example
setting
CMS_NO_CONTENT_VERIFY will totally disable all content
verification and any modified content will be considered valid. This
combination is however useful if one merely wishes to write the content to
out and its validity is not considered important.
Chain verification should arguably be performed using the signing time rather
than the current time. However since the signing time is supplied by the
signer it cannot be trusted without additional evidence (such as a trusted
timestamp).
RETURN VALUES¶
CMS_verify() returns 1 for a successful verification and zero if an error
occurred.
CMS_get0_signers() returns all signers or NULL if an error occurred.
The error can be obtained from
ERR_get_error(3)
BUGS¶
The trusted certificate store is not searched for the signing certificate, this
is primarily due to the inadequacies of the current
X509_STORE
functionality.
The lack of single pass processing means that the signed content must all be
held in memory if it is not detached.
SEE ALSO¶
ERR_get_error(3),
CMS_sign(3)
HISTORY¶
CMS_verify() was added to OpenSSL 0.9.8